1. Field of the Invention
This invention is directed to a filter for a power circuit, and more particularly to a transformer-based magnetic filter for filtering harmonic frequency currents from the power circuit. The invention is also directed to a transformer-based magnetic filter for synthesizing a waveform with minimal harmonic content from multiple plural-phase inputs.
2. Background of Information
Power generation and distribution systems generally include plural power lines (e.g., three phases) which provide alternating current power from a power source (e.g., a generator, a feeder, an inverter, etc.) to a plural-phase load. Each of the plural power lines includes current and voltage waveforms which have a uniform phase shift (e.g., 120.degree.) between sequential phases (e.g., phase A to phase B, phase B to phase C, or phase C to phase A). Each of these waveforms operates at a fundamental frequency (e.g., 50 Hz, 60 Hz, 400 Hz, etc.). The power generation and distribution system typically introduces harmonics of the fundamental frequency and, thus, distorts such current and voltage waveforms.
Development of a Large Static VAR Generator Using Self-Commutated Inverters for Improving Power System Stability, 92 WM 165-1 PWRS, January 1992, discloses a static VAR generator system. The generator system includes eight unit inverters which have 48 output voltages of the same wave shape. The 48 output voltages are offset by a 7.5 degree phase angle. These output voltages are combined by a multiple transformer having 8 secondary windings and a main transformer in order to form a 48 pulse wave shape having reduced output current harmonics.
In a three-phase system having two three-phase inverters which are offset by 30 degrees, it is known to utilize three three-winding transformers between the inverters and a step-up transformer in order to eliminate a particular set of harmonics. However, the step-up transformer requires a fixed 30 degree phase shift and the three-winding transformers require a specific .sqroot.3:1:1 turns-ratio.
There is a need, therefore, for a simplified apparatus which filters harmonic currents from a power distribution system having dual three-phase inverters.
There is a more particular need for such an apparatus which does not require a specific phase shift in a step-up transformer or between the inverters.
There is another more particular need for such an apparatus which does not require a specific turns-ratio in a transformer.
There is yet another more particular need for such an apparatus which reduces the requisite number of transformers.
There is still another need for an apparatus which synthesizes a waveform with minimal harmonic content from multiple plural-phase inputs.